1. Field of the Invention
The present invention relates to an optical pickup and an optical element or device for use therein and, more particularly, to an optical pickup that can be used equally for two optical recording media of different thicknesses and hence different in the distance from the optical pickup held in its playback position to their recording surfaces.
2. Prior Art
A digital video disk (hereinafter referred to as a DVD) has a recording capacity six to eight times larger than those of a compact disk (CD) and a laser disk (LD) now in wide use, and hence it is recognized as a next-generation recording medium.
Such a large recording capacity of the DVD is accomplished by increasing the recording density on the disk and numerical aperture (N.A.) of an objective lens in the playback system. That is, the spot diameter of laser beam irradiated on the disk surface is reduced by using an object lens having large numerical aperture (N.A.), thereby permitting the readout of information recorded in the disk surface with high density.
On the other hand, as the numerical aperture (N.A.) of the objective lens increases, the readout of information becomes more and more affected by aberration or birefringence (double refraction) attributable to the disk thickness. Therefore, a tilt angle, which is an angle between the horizontal plane of the disk and the orthogonal plane with respect to the optical axis of the optical pickup, should be minimized, and the allowed value of the tilt angle becomes small. However, it is impossible to play back the disk according to its warpage.
The influence of the tilt angle with respect to an incidence of an error is expressed by A*d*(N.A.).sup.3, where A is a coefficient and d is the disk thickness. As a solution to this problem, it is general practice in the prior art to decrease the DVD thickness to reduce the optical path over which laser beam enters the disk at its surface and is reflected from the plane of recording of the disk and back to the disk surface so that the desired allowed tilt angle can be retained even when the numerical aperture (N.A.) is large.
To this end, the DVD thickness is set at 0.6 mm, i.e., one-half the thickness t=1.2 mm of a conventional CD and an objective lens of a large numerical aperture is used to reduce the spot diameter of laser beam so as to accomplish a stable readout of data.
Incidentally, the CD has already become widespread as a disk-shaped storage medium. Since the DVD and the CD (including a video CD) are common in outer dimensions and are similar in the manner of use, that is, since the DVD is expected to be used in place of CD as a optical recording media having high sound quality, there is a demand for a DVD/CD-compatible playback system equipped with both DVD and CD playback functions.
When a CD and a DVD, each having different thicknesses are played back by the same reproducing device with an ordinary optical system, the spot diameter of laser beam on the CD with the larger thickness becomes too large to read out data therefrom. To solve this problem, there have been proposed so far various methods that enable the same playback system to be used equally for playing back two optical recording media of different thicknesses.
A first one of the conventional methods employs two optical pickups for a 0.6-mm thick disk (DVD) and for a 1.2-mm thick disk (CD) in the playback system and switches them from one to the other according to the kind of the disk to play back. With this method, however, the playback system is inevitably bulky and costly.
A second method employs two objective lenses for the DVD and for CD provided in the optical pickup and switches them from one to the other according to the kind of the disk to play back. This method also has a defect that the playback system becomes bulky.
A third method is to implement two focal lengths by one optical pickup as disclosed in Japanese Pat. Laid-Open Gazette No. 311945/95. This is shown in FIG. 8, in which the power of a light source 1 is selectively switched to emit therefrom an optical beam 17a or 17b of a different diameter for incidence on an objective lens, thereby creating an optical beam of a different focal length 18a or 18b. With this method, the playback system can be made smaller than in the cases of the first and second methods mentioned above.
Since the third method depends largely on the light source, however, it has a disadvantage that the desired beam diameter may sometimes be unobtainable due to aging of the light source. In such an instance, an error will arise in the focal length, changing the optical spot diameter on the disk accordingly, and hence introducing a readout error.